The present invention generally relates to AC induction motors and controllers therefor. More specifically, the invention relates to drives for such motors. Yet more specifically, the invention relates to variable speed drives for such motors.
Planers are machines used to cut workpieces, and more specifically, to provide flat surfaces thereon. In one application, in the paper making industry, planers are used to cut steel headbox parts and other components for pulp processing machines. Examples of such planers include GRAY planers, CC1160 and CC1170.
These planers have continuously been employed in the paper making industry since the 1950s. As such, the controls therefor date back to the 1950s as well. Yet, the drives therefor are relatively inefficient and subject to relatively quick wear and relatively expensive maintenance. Moreover, motors controlled by these drivers exert variable torque, depending on the speeds at which the motors are driven.
In other applications, it is known to utilize an adjustable frequency motor drive system in which the frequency of the power supply to the motor can be selectively varied and the speed of the motor will follow the varying frequency at a slip speed relative to the frequency. A specific example includes drive systems for hoists. Patent documents relating to such drives include U.S. Pat. Nos. 4,953,053; 4,965,847; 5,027,049; and 5,077,508, the disclosures of which are incorporated herein by reference.
In these patents, there is described an adjustable frequency power supply interposed between a mains switch and the motor to be driven. Sixty hertz supply power is input through the mains switch to the adjustable frequency power supply which in turn provides power to the motor. Additionally, a DC power supply is provided to a controller through a transformer connected across a portion of the input power supply lines. A signal converter is connected between the motor and the adjustable frequency power supply for use in motor rotation direction and speed deviation detection purposes.
An alternating current three-phase power supply is provided to the motor from the adjustable frequency power supply. The rotational direction of the motor is determined by the phase sequence of the three-phase power supply thereto.
The adjustable frequency power supply includes an inverter, a microcomputer and an EPROM all connected together by a bus. Information in digital signal form is transferred between the microcomputer, the EPROM and the inverter on the bus. The microcomputer is also connected to the controller for transmitting information signals directing the control of the microcomputer and the controller and to an overload display. The microcomputer includes a microprocessor, a memory and input and output units.
The EPROM contains a program for controlling the operation of the motor in conjunction with signals received by the microcomputer from the controller, the signal converter, and the inverter. The inverter receives the three-phase 60 hz input power and outputs a three-phase selectively variable frequency output to the motor.
As is known, in the inverter, the three-phase input power is rectified to full wave direct current power and then converted to three-phase alternating current (square wave) power at a constant voltage to frequency ratio and at a frequency which may be controlled by signals from the microcomputer. The phase sequence of the alternating current supply to the motor which controls the direction of rotation of the motor is directed by a signal from the controller to the microcomputer.